1. Field of the Invention
The present invention relates to a technology for recovering a failure in a network that shares a part of a plurality of rings and avoids occurrence of a loop path by providing a block in each of the rings.
2. Description of the Related Art
Recently, a network based on layer 2 using Ethernet (TM) is often used for building a large scale network such as a wide area network. The network using Ethernet (TM) has such advantages that its introduction cost is comparatively low but high-speed communication can be achieved.
When a large scale network is built using Ethernet (TM), how to ensure redundancy is a significant matter. Because Ethernet (TM) has been developed specifically for a small scale network such as a local area network (LAN) and has therefore no redundancy that is required for the large scale network.
One of technologies of providing redundancy to the network based on Ethernet (TM) is Ethernet (TM) Automatic Protection Switching (EAPS). This is a technology specific to a loop topology, and high-speed failure recovery is thereby achieved. EAPS selects one of data relay apparatuses, as a master, connected to one another in a ring shape (a data relay apparatus other than “master” is called “transit”), and suppresses occurrence of a loop of packets by blocking one port by the master. The network is restored by releasing the port upon occurrence of a failure (see S. Shah, M. Yip, “Extreme Networks' Ethernet (TM) Automatic Protection Switching (EAPS) Version 1”, [online], October 2003, retrieved from the Internet:
<URL:http://www.ietf.org/rfc/rfc3619.txt>).
Japanese Patent Application 2004-076593 discloses a technology of improving EAPS. This technology is used when a failure occurs, and it is used so as to prevent occurrence of communication interruption upon failure recovery by re-selecting a data relay apparatus, as a master, provided adjacent to a location where the failure occurs. PCT/JP2005/000143 discloses a technology of efficiently performing the process of re-selecting a master in Japanese Patent Application 2004-076593.
In EAPS version 2 (EAPS v2) that is extended specifications of EAPS, redundancy in a multi-ring structure is also ensured. When a plurality of loop-type networks are caused to have a shared portion to form a multi-ring structure, so-called a super loop may be formed if a failure occurs in the shared portion, and a loop of packets may stop the network.
To solve the problems, in EAPS v2, a role as a controller or as a partner is given to the data relay apparatus in a shared portion. If a failure occurs in the shared portion, then the controller keeps only one of ports on the opposite side to a location where the failure occurs, and suppresses formation of the super loop by blocking other ports (see “Solution Guide”, [online], retrieved from the Internet:
<URL:http://www.extremenetworks.co.jp/solutions/enterprise_solution/solution_guide/index.htm>).
In the system of EAPS v2, however, it is necessary to control data relay apparatuses by using their roles that includes four types in total obtained by adding roles of a controller and a partner to the master and the transit. Therefore, the load for the control increases. Furthermore, a position of the block largely changes when the failure occurs and the failure is recovered, which causes communication interruption to occur over the whole network.